1. Field of the Invention
The invention relates to a fuel cell system and a method of controlling the fuel cell system. In particular, the invention relates to a fuel cell system that controls the output voltage of a fuel cell by means of a DC/DC converter, and a control method of the fuel cell system.
2. Description of the Related Art
A conventional fuel cell generates electric power through an electrochemical reaction of hydrogen and oxygen in air. The fuel cell is a clean electric power generation unit that does not discharge carbon dioxide, which is considered to be a cause for global warming. The fuel cell is expected as an electric power supply unit for an electric vehicle employing a motor as a motive power source.
In general, a fuel cell is constructed as a cell stack that includes a multitude of unit fuel cells, each of which generates electricity, connected in series and laminated on one another. Each of the unit fuel cells may be constructed by, for example, clamping a solid polymer electrolyte membrane between an anode-side electrode and a cathode-side electrode and then clamping this assembly between two separators, one of which has a hydrogen flow passage and the other an air flow passage formed therein.
Each of the electrodes has a catalytic layer that contacts the electrolyte membrane, and a gas diffusion layer that is formed on a surface of the catalytic layer. The catalytic layer consists mainly of carbon powder carrying a metal catalyst that includes, for example, platinum. Further, the gas diffusion layer air permeable and electrically conductive.
In unit fuel cell described above, the hydrogen supplied to the anode-side electrode discharges electrons due to an activation effect of the catalytic layer, and thereby turns into hydrogen ions (i.e., protons). The hydrogen ions permeate the electrolyte membrane, which exhibits good ion conductivity in a wet state, and move to the cathode side. Further, the electrons discharged in being turned into the hydrogen ions are taken out from the anode-side electrode of each of the unit fuel cells, collected, and output as an electric power generated by a fuel cell stack. In contrast, the oxygen in air supplied to the cathode-side electrode takes in the electrons, which have been recirculated to the cathode-side electrode of each of the unit fuel cells from outside the stack due to the activation effect of the catalytic layer, and thereby turns into oxygen ions. The oxygen ions then form an electrochemical union with the hydrogen ions that have permeated the electrolyte membrane, thereby producing water. The water thus produced is discharged, together with the air discharged from each of the unit fuel cells, from the fuel cell stack via a manifold.
It is known that when a state of an open circuit voltage (OCV) as a maximum possible output voltage is held in the fuel cell having the electric power generation function as described above, the platinum catalyst elutes and deteriorates. Thus, the supply of hydrogen and air to the fuel cell is, for example, adjusted or stopped so as to perform electric power generation operation at a voltage equal to or lower than an maximum operating voltage lower than the open circuit voltage.
In this case, in a vehicle mounted with a fuel cell system, when the operation of the fuel cell system is stopped in response to the manipulation of a switch by a user, it is conceivable to stop supplying the fuel cell with hydrogen and air, and shut down electric power equipment for converting or controlling the output voltage from the fuel cell. In such a case, even when the output voltage is controlled to a voltage equal to or lower than the maximum operating voltage lower than the open circuit voltage as described above during the operation of the system, electric power generation is continued due to the hydrogen and air remaining in the fuel cell after the stoppage of the operation of the system. As a result, there arises a problem in that the electromotive force of each unit fuel cell rises to the open circuit voltage.
For example, Japanese Patent Application Publication No. 2005-100820 (JP-A-2005-100820) describes a fuel cell system that supplies a fuel cell only with hydrogen after the termination of normal stop operation, consumes oxygen remaining on an air electrode in the fuel cell, and hence lowers the voltage of the fuel cell. This publication describes that the fuel cell can thus be prevented from being deteriorated by being left in a high-potential no-load state, and that the amount of wastefully discharged hydrogen can be reduced to avoid a deterioration in fuel consumption.
Further, Japanese Patent Application Publication No. 2008-218398 (JP-A-2008-218398) describes that an output voltage of a fuel cell is so controlled as to be held at a high potential avoidance voltage lower than an open circuit voltage through the operation of a DC/DC converter electrically connected to the fuel cell in an electric power generation stop state, in intermittently operating the fuel cell by making a changeover in the state thereof between an electric power generation operation state and an operation stop state.
However, JP-A-2005-100820 only describes that the fuel cell is supplied with only hydrogen after the stoppage of the operation of the system, the oxygen remaining in the fuel cell is consumed, and hence the voltage of the fuel cell is lowered. This publication does not give any concrete description of how to restrain the output voltage of the fuel cell from rising to the open circuit voltage due to the electric power generation occurring in consuming the remaining oxygen.
Further, JP-A-2008-218398 is effective in holding the output voltage of the fuel cell at the high potential avoidance voltage lower than the open circuit voltage during intermittent operation of the fuel cell. However, this publication does not offer any solution to the necessity of restraining the output voltage of the fuel cell from rising to the open circuit voltage after the stoppage of the operation of the system including the fuel cell.